1. Field of the Invention
This invention generally relates to a device for controlling the commutating angle of a separate excitation type inverter device where the A.C. power source is used as a commutating power source such that failures in commutation are prevented in cases when the voltage of the A.C. power source drops or becomes unbalanced.
2. Description of the Prior Art
A conventional static Scherbius device as shown in FIG. 1 has been employed as, for example, a device for controlling the speed of a motor for operating a water pump. In this case, even though the A.C. voltage of the power source drops, or becomes unbalanced, it is required that the operation of the static Scherbius device continue. Therefore, it is necessary to control the commutating angle within some margins such that operation can be continued even though the voltage of the A.C. power source drops or becomes unbalanced. However, it is difficult to realize such a technique by using the prior art static Scherbius device as will be described below.
In FIG. 1, the A.C. power source 1 is connected with a wound-rotor type induction motor 3 through a circuit breaker 2. The electric power from the secondary windings of the induction motor 3 is converted to D.C. power by a rectifier 4 and then the converted D.C. power is reconverted to A.C. power by an inverter bridge circuit 6 which is coupled back to the A.C. power source 1. A smoothing reactor 5 is provided between the rectifier 4 and the inverter bridge circuit 6. Three capacitors 7 are connected to the line of the A.C. power source for power factor improvement. The output of a speed control 8 is compared with the amplitude of a speed detector 9 such as a tachometer, which is coupled with the shaft of the induction motor 3. The compared signal is supplied to a current amplifier 10 which produces a current reference signal I.sub.R. The current reference signal I.sub.R of the current amplifier 10 is compared with the output of a current detector 11 which is connected between the rectifier 4 and the inverter bridge circuit 6. The compared signal is supplied through a phase amplifier 12 to a phase controlling circuit 13 which receives a synchronous signal from the A.C. power source 1 through a transformer 14 and which produces gate signals for each gate of the thyristor SCR, through SCR.sub.6 of the inverter bridge circuit 6.
Consequently, the phase angle of the thyristors of the inverter bridge circuit 6 is controlled, and a so-called speed controlling circuit having a current control sub-loop is constituted. The portion of the circuit relating to the commutation function which operates when the input power source drops or becomes unbalanced corresponds to the phase controlling circuit 13. The phase controlling circuit 13 is explained below in detail with reference to FIG. 2.
Referring to FIG. 2, the secondary of the transformer 14 is constituted as a six-phase connection and is connected with a filter comprising a plurality of resistors 21 and a plurality of capacitors 22 for producing a 30.degree. delay in phase angle and for the elimination of distortions and noise from the waveform of the A.C. power source. Control of the amount of lead in the phase angle (.alpha.) is achieved by adding a sinewave signal from the A.C. power source and a controlling voltage.
In the phase controlling circuit 13 of FIG. 2, only the circuitry for one phase is shown. The two voltage signals b and e are added together in a ratio determined by the value of the resistors R.sub.1 and R.sub.2. The sum signal is supplied to a level detector 23 which detects a predetermined level, for example the zero point. The output signal of the level detector 23 is supplied to an AND circuit 27 which acts as a limitation of the amount of lead of the phase (the so-called .alpha.-limit). The output signal of the phase amplifier 12 which determines the controlled phase angle and the voltage signal c from the secondary of the transformer 14 are added through resistors R.sub.3 and R.sub.4, respectively. The added output signal is supplied to an input terminal of an OR circuit 26 as a leading phase angle (.alpha.)controlling signal through a level detector 24 which detects a predetermined level, for example the zero point.
On the other hand, the voltage signal f is supplied to the outer input terminal of the OR circuit 26 which functions to limit the amount of lag of the phase angle (the so called .beta. limit) through a resistor R.sub.5 and a level detector 25 which detects a predetermined level, for example, the zero point. The output signal of the OR circuit 26 is supplied to the other input terminal of the AND circuit 27. The output signal of the AND circuit 27 is converted to a pulse signal by a one-shot circuit or monostable multivibrator circuit 28. The output signal of the one-shot circuit 28 is supplied to an input terminal of an OR circuit 29. Similarly, the pulse of the next phase is obtained from a one-shot circuit or monostable multivibrator circuit 30. Thus, two output signals of the one-shot circuits 28 and 30 are supplied to the gate of the thyristor SCR.sub.3 in the inverter bridge circuit 6 through the OR circuit 29.
FIG. 3 shows the relationship between each phase shown in FIG. 2. The control of the amount of lead in the phase angle (.alpha.) is achieved by adding the output signal of the phase amplifier 12 and the voltage signal c. Accordingly, the leading phase angle (.alpha.) control signal is changed within the period from time t.sub.2 to t.sub.4 in response to the amplitude of the output signal of phase amplifier 12. But the angle of lead, .alpha., is limited to a range of from 10.degree. to 20.degree. by adding the output signal of level detector 23 and the output signal of OR circuit 26. Occasionally, the commutating voltage is maintained in which the phase leads at a maximum angle. On the other hand, in the case where the amount of lead of the phase angle (.alpha.) is controlled at the delayed time point t.sub.4, the angle of lag .beta., is limited so as to maintain the angle of lag to less than 20.degree. or 30.degree.. Occasionally, the commutating voltage is maintained in which the phase lags at a minimum angle.
The .beta. limiting (limiting the angle of lag), when the inverter circuit is operated, is a property determined by the impedance of the A.C. power source and the load circuit. But in general the angle of lag, .beta., is limited to range between 20.degree. to 40.degree. when the inverter is operating. However, in the case where the voltage of the A.C. power source drops abnormally, as for example, when the voltage of the phase which is conducting drops to an abnormally low voltage, as mentioned above, if the thyristor is turned on with the same phase angle of lag, .beta., as that of the normal state or condition, the commutating completion time becomes too short due to the dropped voltage. Occasionally a failure in in commutation will occur.
The operation in which commutation is achieved from the phase U to the phase V at the time t.sub.1 of FIG. 3 will now be explained. Namely, the commutating completion time, in which the inverter bridge circuit 6 is commutated from the point B to the point A, is determined by a time-product of the voltage difference between points A and B shown in FIG. 3 and the voltage difference thereafter, as for example, the oblique portion (A-B-C) shown in FIG. 3.
If the voltage UW drops abnormally to a low voltage at the point A', since the phase angle of lag, .beta., is fixed at the time t.sub.1, the time-product of the commutating voltage becomes long and does not finish by the point C'. Consequently, a failure in commutation will occur. Furthermore, in the case where the A.C. power source becomes unbalanced, since the .beta. limit angle of that phase is determined by the phase angle of another input phase, occasionally a reduction in the .beta. limit angle will occur.